Machine vision relates to automated product inspection and is generally is used for quality inspection during product manufacturing. The basic elements used in a machine vision system are similar to those employed by a human inspector, namely: 1) a sensor (i.e., the eye) to take a picture of the object under inspection; and 2) a comparator (i.e., the brain) which compares the picture formed by the sensor against a known reference. Thus, in its simplest form, a machine vision system compares a picture of what it sees against a known reference.
A typical system of this type includes a machine vision controller which controls the operation of a video camera and receives analog video data from the camera. The controller also provides control signals to control electronics which drive a light source or backlight. As the object to be inspected moves on a conveyer belt, for example, the object is illuminated with light from the light source, and the camera captures an image of the illuminated object. The camera then transmits a signal indicative of the image to the controller which compares the image against the known reference. If a captured image is outside a certain predetermined tolerance in comparison to the known reference, the object under inspection fails the inspection.
In recent years, light emitting diodes (LEDs) have become the dominant light sources in machine vision systems due primarily to the relatively small size, long life and fast switching speed which allows them to be used in a strobed mode or application. In the past, there has been no control over the direction of the beam of light emitted from each LED in the array. This is because, due to manufacturing tolerances, most LEDs are not ideal ones. As shown in FIG. 1A, the LED may be slightly out of alignment so that the light beam B emitted by the LED is not symmetrical about its optical axis O, i.e., it deviates therefrom as shown by angle α1, in that figure. To overcome that problem, a technique was devised for aiming, while mounting, even non-ideal LEDs in an array to compensate for such deviations such that the array provides a relatively uniform output; see U.S. Pat. No. 5,822,053, the contents of which are hereby incorporated herein by reference.
In accordance with that patent, the light source includes a housing with a rigid base plate. Each LED L (FIG. 1A) of the array is situated in a hole in the base plate and pointed such that the light emitted by the LED illuminates a known location. Each LED is then secured by a UV-curable cement such that it remains in its predetermined position.
While the mounting technique disclosed in that patent is suitable for mounting and aiming the relatively low-intensity (e.g. 20-60 milliwatts) LED L having electrical leads that suffice to conduct away heat from the LED, it cannot be used with the high intensity (e.g., 1-5 watts) LED HIL shown in FIG. 1B preferred for use in today's machine vision systems because of its superior radiation characteristics and longer operating life. The latter LED HIL has a current draw of 350-750 milliamps and thus generates much more heat than the conventional LED L. As such, the high intensity LED HIL requires a large-area heat sink slug which must be in intimate thermal contact with the light sourcebase plate and/or housing in order to conduct away that heat. This means that such high-intensity LEDs cannot be anchored in holes in the housing base plate and be pointed in a predetermined manner as described in the above patent.